This invention is related to acousto optical diffraction cells, which have been used for some time as light beam scanning and modulating devices in systems utilizing monochromatic light sources such as lasers.
Diffraction cells of various types and with a wide range of performance characteristics are manufactured by FJW Industries (Model M-40R Light Modulator, Model D-70R laser Deflector and others) and by Isomet Corporation (Series LD-401 and others).
A common limitation for all models is the inability to properly control polychromatic light.
Diffraction cells are usually operated in such a way, that the Bragg condition is satisfied. This requires, that ##EQU1## where the Bragg angle, B is the angle between the ultrasonic wavefront and the direction of light inside the crystal, n is the refractive index of the crystal at a given value of w, which is the wavelength of light in vacuum, and W is the wavelength of the ultrasonic wave inside the crystal.
Not only will light of different wavelengths be deflected through different angles for the same value of W but the efficiency of the cell, defined as the percentage of light, that can be removed from the zero order beam, decreases as the input angle deviates from the Bragg angle.
Temperature changes in the diffraction cell material have an effect on ultrasonic wavelength and therefor on deflection angle. U.S. Pat. No. 3,462,212 illustrates an attempt to overcome some of these problems. It should be realized however, that the ability to deflect a polychromatic beam, such that color integrety is preserved in the exit beam, is possible only for one particular deflection angle, and even in that case only if the index of refraction of the cell material is a linear function of the wavelength of light.
It is therefor an object of this invention to provide a light beam scanning device, which will afford true linear scanning of a polychromatic light beam.
It is an other object of the invention to provide an acousto optical diffraction device, that will maintain color integrety of the scanning light beam.
It is a further object of the invention to provide an acousto optical diffraction device, useful for modulation of polychromatic light.
It is a further object of the invention to provide a light beam scanning device, where the light beam direction is independent of device temperature.
It is additionally an object of the invention to provide a polychromatic light modulator, that can be used with a high intensity light beam.
It is also an object of the invention to provide a linear scan device, that can be operated in either the positive or the negative mode.
It is a further object of the invention to provide a lightbeam scan device, where the output beam direction is independent of the refractive index of the device material.
It is an additional object of the invention to provide a polychromatic scan device, that will operate at high scanning rates.